The invention relates to making composite material parts with fiber reinforcement and a resin matrix by using a method of injecting resin under pressure also referred to as resin transfer molding (RTM).
The field of application of the invention is more particularly that of making thick parts out of composite material. As examples of such parts, mention can be made of the diverging portions of rocket engine nozzles such as those made of composite material having carbon fiber reinforcement and a phenolic resin matrix. Naturally, the invention is applicable to fabricating a wide variety of parts, whether parts for rocket engines or airplane engines, or parts suitable for use more generally in the fields of aviation and space, or in other fields.
A technique commonly used for making thick parts of composite material consists in using a resin to pre-impregnate layers or strips of woven fabric or other fiber textures, in draping or winding the pre-impregnated layers or strips on a shaper and a mandrel until a desired thickness is obtained, then covering the resulting blank in a delamination film, a resin drain fabric, and an elastomer membrane in order to allow the resin to be polymerized in an autoclave and thus obtain a part having substantially the desired shape.
Such a method makes it possible to achieve technical results that are satisfactory for certain potential applications, i.e. low residual porosity and a reinforcing fiber fraction that is quite high. Nevertheless, implementing that method industrially presents drawbacks: resin impregnation and polymerization within an autoclave after draping or winding are implemented as a plurality of successive steps. The operation of impregnation by passing through baths implies using solvents and requires special treatment of effluents since they present problems in terms of the environment, hygiene, and safety.
The RTM method has also been known for a long time and it is in widespread use, enabling the steps of impregnating a fiber reinforcement in a mold by injecting resin to be followed immediately by polymerization in an autoclave without the impregnated fiber reinforcement being left in open air.
Nevertheless, if a conventional RTM method is implemented on fiber reinforcement of great thickness, it is difficult to obtain composite material parts presenting little residual porosity. In order to be able to impregnate thick fiber reinforcement right through to the core it is necessary for the resin to present low viscosity. Lowering viscosity by using solvents, and using resins that give off volatile materials during polymerization, as applies in particular for phenolic resins, mean that a high level of residual porosity is present in the composite material after the resin has been polymerized. It is indeed possible to reduce porosity by repeating the impregnation and polymerization cycle several times, but only with significantly increased durations and processing costs.